JPH0612778B2 - Semiconductor device - Google Patents

Description

The present invention relates to a semiconductor device capable of high speed operation.

[Conventional technology]

There is one heterojunction bipolar transistor (HBT) with a wide bandgap emitter (WGE) in active semiconductor devices that is believed to be capable of high speed operation. For example, Asbeck et al. International Electron Device Meeting (IEDM,
Technical digest, p. 629, 1981), a prototype of HBT is reported. This device (1) can significantly reduce the base resistance and narrow the base width without deteriorating the emitter injection efficiency, and (2) can reduce the impurity concentration in the emitter region, thereby reducing the emitter-base capacitance. Since it has the advantage of, it is more suitable for high-speed operation than an ordinary bipolar transistor consisting of a homojunction.

FIG. 6 shows a schematic cross-sectional view of a bipolar transistor having a conventional structure. In FIG. 6, 1 is a semiconductor substrate, 2 is a collector layer made of a first semiconductor having one conductivity type, 3 is a base layer made of a first semiconductor having a conductivity type different from that of the collector layer, and 4 is a collector. An emitter layer 5 made of a second semiconductor having the same conductivity type as that of the layer 2 and having a wider band gap than the collector layer 2 and the base layer 3 is a collector electrode which forms ohmic contact with the substrate 1 and the collector layer 2, and 6 is Reference numeral 7 is a base electrode forming an ohmic contact with the base layer 3, and reference numeral 7 is an emitter electrode forming an ohmic contact with the emitter layer 4.

The operation of this conventional structure is performed by using the semiconductor substrate 1 as n ⁺ -GaAs having a donor concentration of about 1 × 10 ¹⁸ cm ⁻³ and the collector layer 2 as n ⁻ -GaAs having a donor concentration of about 1 × 10 ¹⁶ cm ⁻³ .
The base layer 3 is p ⁺ -GaAs having an acceptor concentration of about 1 × 10 ¹⁹ cm ⁻³ , and the emitter layer 4 is a donor concentration of 5 × 1.
An explanation will be given with reference to FIG. 7 showing this band structure using n-Al _0.2 Ga _0.8 As of about ¹⁷ cm ⁻³ .

FIG. 7 shows a schematic band structure extending over the emitter layer 4, the base layer 3 and the collector layer 2 of FIG.
In FIG. 7, E c is the conduction band edge, Ev is the full band edge, and E f
Is the Fermi level, Veb is the voltage between the emitter and base, V
bc is the base-collector voltage.

When a forward bias of Veb is applied between the emitter and the base and a reverse bias of Vbc is applied between the base and the collector,
Electrons are injected from the emitter to the base by diffusion, and most of the electrons move to the collector side by diffusion in the base layer,
It reaches the collector by being accelerated by a strong electric field in the depletion layer between the base and the collector. Since the amount of electrons injected from the emitter to the base changes with Veb, the collector current is controlled by the base voltage. In a bipolar transistor that has only a normal homojunction, when injecting electrons from the emitter to the base, holes are injected from the base to the emitter, so the emitter injection efficiency (the ratio of the electron current to the emitter current) decreases. To do. However, in the HBT, since there is an Al _0.2 Ga _0.8 As / GaAs hetero interface between the emitter and the base, there is a barrier of about 100 meV for holes when looking at the emitter side from the base side, and there is a barrier from the base to the emitter. The injection of holes is controlled. Therefore, the hole concentration of the base can be increased and the emitter electron concentration can be suppressed to some extent low without lowering the emitter injection efficiency. As a result, the base resistance is small, the emitter-base capacitance is small, and the base width is narrow, so that the structure suitable for high-speed operation can be obtained.

[Problems to be solved by the invention]

However, although the conventional HBT has the above-mentioned advantages, sufficient speedup has not been achieved because it still includes elements that impede speedup.

One of the factors that impedes speeding up is the structure of the base. As a result of containing a high concentration of impurities, the migration rate of minority carriers decreases due to impurity scattering, and the lifetime of minority carriers decreases due to an increase in recombination centers. In addition, since minority carriers move in the base by diffusion, the base transit time increases as the temperature decreases, and the operating speed at low temperatures is slow.

An object of the present invention is to eliminate the drawbacks of the conventional HBT and provide a semiconductor device capable of operating at an ultrahigh speed.

[Means for solving problems]

A semiconductor device of the present invention has a collector layer made of a semiconductor having one conductivity type, a base layer made of a semiconductor having a conductivity type different from that of the collector layer, and a collector layer having the same conductivity type as that of the collector layer. In a structure having an emitter layer made of a semiconductor having a large band gap, the emitter layer has a thickness that does not contain impurities between the base layer and the emitter layer and carriers from the emitter layer cannot escape to the base layer by a tunnel effect. It is characterized by including an emitter barrier layer made of a semiconductor having a wider bandgap than that of the semiconductor.

At this time, when the emitter layer is an n-type semiconductor, the conduction band edge energy of the emitter barrier layer is higher than the conduction band edge energy of the base layer, and when the emitter layer is a p-type semiconductor, the filling band of the emitter layer. The edge energy is lower than the filling band edge energy of the base layer.

[Action]

In the semiconductor device of the present invention, the minority carriers injected from the emitter layer into the base layer through the emitter barrier layer are accelerated due to band discontinuity between the emitter barrier layer and the base layer and have high energy. Therefore, ultra high speed operation is possible.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to the drawings illustrating an embodiment.

FIG. 1 is a schematic sectional view showing a first embodiment of the present invention. In FIG. 1, those having the same numbers as those in FIG. 6 are equivalent to those in FIG. 6 and perform the same functions. Reference numeral 8 denotes an emitter barrier layer which does not contain impurities of a donor and an acceptor and has a thickness such that electrons cannot escape due to a tunnel effect. As various examples of the first embodiment, the emitter barrier layer 8 is made of undoped Al _0.4 Ga _0.6 As and has a thickness of 100 Å, and the others are the same as the above-mentioned conventional example.

The difference between the operation of the first embodiment and the conventional example will be described with reference to FIG. 2 showing the band structure. Figure 2 shows the first
In the figure, the emitter layer 4, the emitter barrier layer 8, the base layer 3,
It shows a schematic band structure over the collector layer 2. ΔE _ceb is the conduction band edge energy difference between the emitter barrier layer 8 and the base 3, and ΔE cee is the conduction band edge energy between the emitter layer 4 and the emitter barrier layer 8.

When a forward voltage V eb between the emitter and the base is applied, electrons tend to flow from the emitter to the base due to diffusion.
However, while Veb is small, injection does not occur because the barrier of ΔEcee exists. Veb becomes big enough,
When ΔEcee is effectively reduced by the tunnel effect,
For the first time, electrons will flow into the base. Since the electrons injected into this base have an energy of ΔEceb or more, they can escape through the base at high speed as hot electro. Even in the conventional structure,
Electrons were injected with extra energy due to the existence of spikes between the bases, but the electrons were injected with V
Even when eb is small, it can be easily tunneled out, and it cannot be a hot electron with large energy.

In the structure of the present invention, most of the electrons injected from the emitter are hot (at an energy position higher than the conduction band edge), so the probability of trapping in the recombination center is reduced and the minority carrier lifetime is reduced. The decrease is also suppressed.

As described above, according to the structure of the present invention, the base running time can be shortened and the minority carrier lifetime can be prevented from being reduced, even though the impurity concentration of the base is high. As a result, it has a high current amplification factor and enables ultra-high speed operation.

Next, the construction method of the above-described first embodiment will be described. MBE (Molecular Beam Epit
axy), an n-GaAs collector layer 2 having a thickness of 0.5 μm and a donor concentration of 1 × 10 ¹⁶ cm ^-3 on an n ⁺ -GaAs substrate 1, a thickness of 500 Å and an acceptor concentration of 2 × 10 ¹⁹ cm ^-3. P ⁺ G
aAs base layer 3, 100 Å thickness of undoped Al _0.4 Ga
_0.6 As Emitter barrier layer 8, donor concentration 5 × 10 ¹⁷ cm
^-3 , an n-Al _0.2 Ga _0.8 As emitter layer 4 having a thickness of 0.5 μm was sequentially grown. The emitter electrode 7 was formed by vapor-depositing AauGe / Au on the surface of the emitter layer 4 and alloying it. The base electrode 6 was formed by removing the emitter layer of the base electrode portion by etching and vapor-depositing AuZn / Au. The collector electrode 5 was In. In HBT by this manufacturing method,
As a delay time per transistor stage, 30 ps was obtained.

FIG. 3 is a schematic band structure diagram of the second embodiment of the present invention. In FIG. 3, those having the same numbers as those in FIG.
It is equivalent to the figure and performs the same function. Reference numeral 9 is a graded base layer in which the forbidden band width gradually expands from the collector layer side, and ΔEb is the difference in the forbidden band width in the graded base layer.

The operation of the second embodiment is almost the same as that of the first embodiment, but the use of the graded base layer enables higher speed operation. Since this graded base layer 9 is p-type, this difference in the forbidden band appears as a difference in energy in the conduction band. Therefore, there is a potential difference corresponding to the band gap difference ΔEb for the electrons, and the electrons are accelerated by the internal electric field in the base. With the above material, the graded base layer 9 is p ⁺ -G
When changing from aAs to p ⁺ -Al _0.1 Ga _0.9 As, the potential difference becomes about 0.12V. Further, if the graded base layer thickness is 1000Å, an electric field of 12 kV will be applied. Therefore, the electrons that have entered the graded base layer 9 from the emitter barrier layer 8 first have a conduction band difference ΔEc (about 0.2 V in the above material) between the emitter barrier layer 8 and the graded base layer 9. By the electric field in the graded base layer. As a result, the base travel time is shortened as compared with the first embodiment.

The graded base layer 9 has a thickness of 500 Å and GaAs to Al on the collector layer side and the emitter barrier layer side.
P ⁺ -Al _v Ga gradually changing to _0.1 Ga _0.9 As
As a result of producing an HBT using _1-x As (p = 2 × 10 ¹⁹ cm ⁻³ ) and the same as in the first example, the transistor 1 was obtained.
A delay time of 25 ps was obtained per stage.

In the first and second embodiments of the present invention described above, npn
Although only shown for HBTs of the type, it is clear that the invention is equally applicable to those of the pnp type with semiconductors of opposite conductivity type.

FIG. 4 is a schematic band structure diagram of the third embodiment of the present invention, in which the conductivity type of the first embodiment is reversed.
In FIG. 4, the same numbers as those in FIG. 2 indicate materials having opposite conductivity types. ΔEveb is the difference in filling band edge energy between the base layer 3 and the emitter barrier layer 8, and ΔEvee is the difference in filling band edge energy between the emitter barrier layer 8 and the emitter layer 4. The operation of this embodiment is the same as that of the first embodiment except that the carriers are holes. A delay time of 35 ps was obtained using the same material and structure as in the first embodiment with the conductivity type reversed.

FIG. 5 is a schematic band structure diagram of the fourth embodiment of the present invention, in which the conductivity type of the second embodiment is reversed.
A delay time of 30 ps was obtained using the same material and structure as in the second embodiment with the conductivity type reversed.

As described above, it is apparent that the present invention can be applied to npn type and pnp type HBTs.

In the above-described embodiments, only the bandgap having a constant band width is shown as the emitter layer 4, but the bandgap may be gradually increased from the emitter barrier layer side. The band gap of the collector layer may be larger than that of the base layer 3. The device structure may be not only a mesa type but also a planar type using ion implantation or regrowth, and the growth order of each layer may be reversed. Although only the MBE method was shown for the growth of each layer, MOCVD (Metal Organic Chemical Vapor D
Other growth methods such as eposition), vapor phase growth method, liquid phase growth method, etc. may be used.

Although only GaAs / AlGaAs system was shown as a semiconductor, GaAs / InGaA using GaAs was also used.
InGaAs / InGaA using sP / InGaP system or InGaAs whose electron saturation speed is higher than GaAs
lAs / InAlAs system, InGaAs / InGaAs
III / V compound semiconductors such as P / InP type and GaSb / AlGaSb / AlSb type, elemental semiconductors such as Ga / SiGe / Si type, II-VI compound semiconductors of CdTe / CdZnTe / ZnTe type, and other various semiconductors It is obvious that the present invention can be applied. The materials shown above are combinations in which the lattice constants are substantially the same, but materials with different lattice constants and strain (for example, InGaAs / InAlGaAs / AlGaAs).
The present invention can also be applied to systems).

〔The invention's effect〕

With the semiconductor device of the present invention, the base delay time is significantly reduced, and ultra-high speed operation becomes possible.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a first embodiment of the present invention, FIG. 2 is a band structure diagram of the first embodiment, FIG. 3 is a band structure diagram of the second embodiment, and FIG. Band structure diagram of the third embodiment, FIG. 5 is a band structure diagram of the fourth embodiment, FIG. 6 is a schematic sectional view of a conventional heterojunction bipolar transistor, and FIG. 7 is the transistor of FIG. It is a band structure figure of. 1 ... Semiconductor substrate 2 ... Collector layer 3 ... Base layer 4 ... Emitter layer 5 ... Collector electrode 6 ... Base electrode 7 ... Emitter electrode 8 ... Emitter barrier layer 9 ... Graded base layer Ec ... … Conduction band edge Ev …… Filling band edge Ef …… Fermi level Veb …… Emitter-base voltage Vbc …… Base-collector voltage △ Eb …… Bandwidth difference in graded base layer △ Eceb …… Conduction band edge difference between emitter barrier and base △ Ecee …… Conduction band edge difference between emitter and emitter barrier △ Eveb …… Filling band edge difference between emitter barrier and base △ Evee …… Filling band between emitter and emitter barrier Margin

Claims (3)

[Claims] 1. A collector layer made of a semiconductor having one conductivity type, a base layer made of a semiconductor having a conductivity type different from that of the collector layer, and a forbidden band than the base layer having the same conductivity type as the collector layer. In a structure having an emitter layer made of a semiconductor with a large width, it has a thickness that does not contain impurities between the base layer and the emitter layer and carriers from the emitter layer cannot escape to the base layer by a tunnel effect. A semiconductor device comprising an emitter barrier layer made of a semiconductor having a wide band gap. 2. The semiconductor device according to claim 1, wherein the emitter layer is an n-type semiconductor, and the conduction band edge energy of the emitter barrier layer is higher than the conduction band edge energy of the base layer. 3. The semiconductor device according to claim 1, wherein the emitter layer is a p-type semiconductor, and the filling band edge energy of the emitter barrier layer is lower than the filling band edge energy of the base layer.